Anomalous spectral holes in a fiber-loop circuit undergoing dynamic modulation

We investigate theoretically and experimentally anomalous spectral holes, i.e. the cancellations of certain frequency modes inside a wide frequency comb, in a fiber-loop circuit with an incorporated phase modulator (PM). During the process of phase modulation, the frequency modes couple between each other accompanying with a nonreciprocal phase, which mimics an artificial gauge potential for photons. As the light circulates in the fiber loop, coherent interference effects occur between frequency modes in distinct circulations, which are highly influenced by the artificial gauge potential. By carefully choosing the gauge potential via adjusting the modulation phase, we can coherently suppress a series of frequency components within a broad spectrum and flexibly manipulate the positions, widths and depths of these drops. In the experiment, we achieved a hole position shift with ~50 GHz and a hole width with ~30 GHz while the extinction ratio reaches ~15 dB. For even higher modulation depth beyond the experiment range, the simulation shows that the width of main spectral hole will grow linearly with the increase of modulation depth while the position and depth tend to be stable. The study reveals the anomalous cancellation of frequency modes in a fiber loop, which holds great promises for potential applications in optical communications and signal processing.